Publish Docker Images to GHCR

[omad]

Switch docker image publishing to GHCR.

We can push to both DockerHub and GHCR for a while, the next release, or something like that.

[pjonsson]

It's still untested, but a couple of months ago I made a PR for datacube-docker that should solve this problem for that repository: opendatacube/datacube-docker#171